Display device

ABSTRACT

A display device includes a first plastic substrate including a composite layer having a resin-impregnated fiber fabric, an inorganic barrier layer formed on the composite layer, and a planarizing resin layer formed on the inorganic barrier layer; and a display medium layer formed on the planarizing resin layer-side of the first plastic substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a display device having a plasticsubstrate and a fabrication method therefor, and more particularly,relates to a display device having a fiber-filled plastic substrate.

In recent years, use of liquid crystal display devices and organic ELdisplay devices for portable information terminal equipment has expandedthanks to their features of being thin, light in weight and low in powerconsumption. With this expansion of the use, there have arisen strongrequests that such display devices should be further light and thin andimproved in shock resistance and other properties.

To respond to the above requests, an idea of using a plastic substrateinstead of the conventionally used glass substrate has been proposed.However, many problems to be overcome are present to adopt a plasticsubstrate made of a resin material in a sheet shape as a substrate of adisplay device.

One of the biggest problems is that a plastic substrate is large inlinear expansivity (i.e., coefficient of linear thermal expansion).While the linear expansivity of glass is generally several ppm/° C.,that of even a type of plastic small in linear expansivity is as largeas several tens of ppm/° C. Being large in linear expansivity indicatesthat the variation in size with temperature is large. Using such amaterial, therefore, it is difficult to fabricate drive elements, suchas TFTs, which require high-precision patterning. A plastic substratemay be used as a counter substrate while the conventional glasssubstrate being used as the substrate on which TFTs are formed(hereinafter, this substrate is also simply called a “TFT substrate”).In this case, also, difficulty is found in positioning color filters(and/or a black matrix) formed on the counter substrate with respect topixel electrodes on the TFT substrate.

To reduce the linear expansivity of a plastic substrate to therebyimprove the size stability, there have been proposed methods of forminga plastic substrate using a material having a filler mixed in a resinmatrix (composite material). As used herein, a plastic substrate formedof a composite material is specifically called a “composite substrate”in some cases.

For example, Japanese Laid-Open Patent Publication No. 11-2812(Literature 1) discloses a reflection conductive substrate having acomposite substrate formed by impregnating a glass fiber fabric with aresin and curing the resin.

Japanese Laid-Open Patent Publication No. 2001-133761 (Literature 2)discloses a composite substrate having fibers arranged in lines orstripes in a resin so as to be kept off from one another. According toLiterature 2, the composite substrate having the impregnated fiberfabric (woven fabric) disclosed in Literature 1 has a problem that wovenportions and intersecting portions of fibers of the fiber fabric causedevelopment of minute unevenness on the substrate surface and thisdegrades the display quality. Literature 2 argues that the disclosedarrangement can provide a composite substrate having a flat surface.

As pointed out in Literature 2, a plastic substrate formed using a glassfiber fabric has an uneven surface. As for the plastic substratedisclosed in Literature 2, fabrication of such a plastic substrate isdifficult. Even if the fabrication is successful, it is difficult toreduce the surface unevenness to the level of 100 nm or less, forexample.

To planarize the surface of the plastic substrate, a planarizing filmmay be formed on the uneven surface described above. By forming such aplanarizing film, a flat surface having unevenness reduced to the levelof 100 nm or less can be provided. However, according to examinationsconducted by the inventors of the present invention, the followingproblem occurs. When an inorganic barrier layer is formed on theplanarized surface, unevenness of 100 nm or more develops on the surfaceof the resultant inorganic barrier layer in some cases. The inorganicbarrier layer is formed to improve the barrier properties of the plasticsubstrate against water and/or oxygen in the air.

SUMMARY OF THE INVENTION

An object of the present invention is providing a display device havinga plastic substrate that includes an inorganic barrier layer and isexcellent in surface flatness.

The display device of the present invention includes: a first plasticsubstrate including a composite layer having a resin-impregnated fiberfabric, an inorganic barrier layer formed on the composite layer, and aplanarizing resin layer formed on the inorganic barrier layer; and adisplay medium layer formed on the planarizing resin layer-side of thefirst plastic substrate.

In one embodiment, the display device further includes a second plasticsubstrate placed to face the first plastic substrate via the displaymedium layer, wherein the second plastic substrate is the same inconstruction as the first plastic substrate.

In another embodiment, the display medium layer includes an organicluminescence layer.

In yet another embodiment, the display medium layer is a liquid crystallayer.

In yet another embodiment, the inorganic barrier layer covers the entirearea of the composite layer facing the display medium layer.

The fabrication method for a display device of the present invention isa fabrication method for the display device described above, wherein aprocess for fabricating the first and second plastic substratescomprises the steps of: preparing the composite layer having theresin-impregnated fiber fabric; forming the inorganic barrier layer on asurface of the composite layer by thin film deposition at a firsttemperature; and forming the planarizing resin layer on the inorganicbarrier layer at a second temperature lower than the first temperature.

In one embodiment, the first temperature is 200° C. or more.

In another embodiment, the inorganic barrier layer includes silicondioxide.

In the plastic substrate of the display device of the present invention,the planarizing resin layer is formed on the inorganic barrier layerthat is formed on the composite layer having the resin-impregnated fiberfabric. Therefore, it is possible to prevent occurrence of the problemthat the surface of the inorganic barrier layer becomes uneven in theheating process for forming the inorganic barrier layer on theplanarizing resin layer, and thus a flat surface can be obtained. Whensuch a plastic substrate is used for a liquid crystal display device,for example, the resultant device is free from variations in cellthickness due to unevenness of the plastic substrate and thus canprovide high-quality display.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross-sectional view and a plan view,respectively, diagrammatically showing a resin-impregnated fiber fabric.

FIGS. 2A and 2B are views showing a fabrication process of aconventional plastic substrate 10, which also demonstrate the mechanismof increase of surface unevenness.

FIG. 3 is a view diagrammatically showing a conventional liquid crystaldisplay device 100.

FIGS. 4A and 4B are views diagrammatically showing a fabrication processof a plastic substrate 20 suitably used in an embodiment of the presentinvention.

FIG. 5 is a cross-sectional view diagrammatically showing a plasticsubstrate 40 suitably used in an embodiment of the present invention.

FIG. 6 is a cross-sectional view diagrammatically showing a liquidcrystal display device 200 of an embodiment of the present invention.

FIG. 7 is a cross-sectional view diagrammatically showing an organic ELdisplay device 300 of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned in the prior art description, the present inventors havefound a phenomenon that when an inorganic barrier layer is deposited onthe surface of a planarizing film of a composite substrate having givenflatness, the flatness is lost and unevenness develops on the surface ofthe inorganic barrier layer. This phenomenon will be described withreference to FIGS. 1A, 1B, 2A and 2B.

FIGS. 1A and 1B are a cross-sectional view and a plan view,respectively, diagrammatically showing part of a resin-impregnated glassfiber fabric 3.

The resin-impregnated glass fiber fabric 3 shown in FIGS. 1A and 1Bincludes a glass fiber fabric 1S and a resin layer 2 covering the glassfiber fabric 1S. The glass fiber fabric 1S is a plain-woven fabric offiber bundles 1A each composed of a plurality of glass fibers 1. Bydipping the glass fiber fabric 1S in an epoxy resin, for example, theresin layer 2 is formed as a thin film covering the surfaces of theglass fiber bundles 1A. As diagrammatically shown in FIG. 1A, theresin-impregnated glass fiber fabric 3 has projections and depressionscorresponding to woven portions at which the fiber bundles 1A intersecteach other.

For example, the diameter of each of the fibers 1 constituting eachfiber bundle 1A is about 10 μm, the width of the fiber bundle 1A isabout 200 μm, and the pitch of the fiber bundles 1A is about 500 μmvertically and horizontally. The thickness of the resin layer 2 is about20 μm, for example. The resultant resin-impregnated fiber fabric 1S hasunevenness (undulation) of the level of about 500 nm to 600 nm.

Conventionally, as shown in FIG. 2A, a planarizing resin layer 14 isformed for planarizing the uneven surface of the resin-impregnated glassfiber fabric 3. The unevenness of the surface is reduced to the level ofabout 100 nm by providing the planarizing resin layer 14.

On the planarizing resin layer 14, an inorganic barrier layer 15 isformed to improve the barrier properties against water and/or oxygen inthe air. The inorganic barrier layer 15, having a thickness of about 100nm, is formed by depositing SiO₂ in a heating film formation process(vacuum evaporation at 200° C., for example). With this formation, thesurface of the resultant plastic substrate 100 (surface of the inorganicbarrier layer 15) has unevenness increased to the level of about 400 nm,as diagrammatically shown in FIG. 2B.

FIG. 3 is a view diagrammatically showing a liquid crystal displaydevice 100 using the plastic substrate 10 described above, in which aliquid crystal layer 16 is sandwiched between the plastic substrate 10and a substrate (glass substrate, for example) 17. In the liquid crystaldisplay device 100, a nonuniform distribution arises in the thickness ofthe liquid crystal layer 16 (cell thickness) under the effect of theuneven surface of the plastic substrate 10. The nonuniform thicknessdistribution of the liquid crystal layer 16 causes a nonuniformdistribution of retardation, and this is visually observed as displayuneveness.

The present inventors examined the mechanism of redevelopment ofunevenness on the once-planarized surface based on various experimentresults, and reached the following conclusion.

During the deposition of SiO₂ on the planarizing resin layer 14 shown inFIG. 2A, the substrate (that is, the resin-impregnated fiber fabric 3and the planarizing resin layer 14 formed thereon) is heated to 200° C.,for example. Since the substrate has regions different in the volumefraction of the fibers 1 from one another, a nonuniform distributionarises in the thermal expansion amount. Specifically, the thermalexpansion amount is small in regions having a larger number of fibers 1,while it is large in regions having a smaller number of fibers 1.Naturally, the thermal expansion amount is largest in regions having nofibers 1. With this nonuniform distribution of the thermal expansionamount, unevenness develops on the surface of the substrate (surface ofthe planarizing resin layer 14). When SiO₂ is deposited on this unevensurface, the surface of the resultant SiO₂ film (inorganic barrierlayer) is also uneven reflecting the unevenness of the underlying layer.The SiO₂ film is small in thermal expansion coefficient and rigid (highin modulus of elasticity) compared with resin. Therefore, the unevennesson the surface of the substrate is retained even after the filmformation process is terminated and the temperature returns to roomtemperature.

In consideration of the mechanism described above that the level ofunevenness increases by forming the inorganic barrier layer 15 on theplanarizing resin layer 14 having a flat surface (unevenness: 100 nm orless), it is understood that if the inorganic barrier layer 15 is formedfirst and the planarizing resin layer 14 is formed on the inorganicbarrier layer 15, a surface having unevenness of 100 nm or less can beobtained.

A plastic substrate suitably used for a display device of an embodimentof the present invention and a fabrication method for such a plasticsubstrate will be described with reference to FIGS. 4A, 4B and 5.

Referring to FIG. 4A, a resin-impregnated fiber fabric 3, which can bethe same as the conventional one described above, is prepared. Aninorganic barrier layer 25 is then deposited on the resin-impregnatedfiber fabric 3. The material and the deposition method for the inorganicbarrier layer 25 may be the same as those conventionally used. A SiO₂film having excellent barrier properties is preferably used as theinorganic barrier layer 25. The water vapor transmission of the SiO₂film (thickness: 100 nm) is 0.4 g/m²/day. When formed at roomtemperature, however, the siO₂ film is poor in chemical resistance, andthis may cause a problem in a TFT fabrication process, for example.Therefore, the deposition is preferably made under heating, specificallyat 200° C. or more. The thickness of the inorganic barrier layer 15 ispreferably about 20 nm or more from the standpoint of the barrierproperties and about 300 nm or less from the standpoint of maintainingthe flexibility of the substrate. SiN_(x), PSG and the like may be usedin place of SiO₂.

A planarizing resin layer 24 is formed on the thus-deposited inorganicbarrier layer 25. The planarizing resin layer 24 may be formed of thesame material in the same manner as those conventionally adopted. Forexample, the same material as that of the resin layer 2 of theresin-impregnated fiber fabric 3 may be used. Such a resin material ispreferably selected to have a refractive index roughly identical to thatof the glass fibers 1 and SiO₂. Also such a resin should preferably beexcellent in chemical resistance and heat resistance. For example, anepoxy resin may be used. The planarizing resin layer is preferablyformed at a temperature lower than the temperature used for formation ofthe inorganic barrier layer. Specifically, the temperature is preferably200° C. or less.

The surface of the thus-formed planarizing resin layer 24 has unevennessof 100 nm or less as that of the planarizing resin layer 14 shown inFIG. 2A has. In this case, however, since formation of an inorganicbarrier layer is no more necessary on the planarizing resin layer 24,there is no increase in the level of unevenness unlike the case shown inFIG. 2B. In this way, a plastic substrate 20 with unevenness reduced to100 nm or less is obtained.

A plastic substrate 40 diagrammatically shown in FIG. 5 may also be usedin place of the plastic substrate 20.

In the plastic substrate 40 of FIG. 5, a planarizing resin layer 42 isformed on the resin-impregnated fiber fabric 3 shown in FIG. 4A, aninorganic barrier layer 45 is then formed on the planarizing resin layer42, and further a planarizing resin layer 44 is formed on the inorganicbarrier layer 45. This is like forming another planarizing resin layeron the surface of the plastic substrate 10 of FIG. 2B. That is, when theinorganic barrier layer 45 is deposited on the planarizing resin layer42 under heating, the unevenness of the resultant surface increases.However, by further forming the planarizing resin layer 44 on theinorganic barrier layer 45, a plastic substrate of which the surfaceunevenness is reduced to 100 nm or less is obtained.

Assume herein that the resin-impregnated fiber fabric 3 and theplanarizing resin layer 42 formed thereon constitute a composite layer43. The plastic substrate 40 then includes the composite layer 43, theinorganic barrier layer 45 formed on the composite layer 43, and theplanarizing resin layer 44 formed on the inorganic barrier layer 45. Inthe plastic substrate 20 of FIG. 4B, the composite layer includes onlythe resin-impregnated fiber fabric 3. Thus, a flat surface can beobtained by forming an inorganic barrier layer on the composite layerand then forming a planarizing resin layer on the inorganic barrierlayer.

As an example of the plastic substrate 40, described will be the case ofusing the composite layer 43 composed of the E-glass fiber fabric (fiberdiameter: 10 μm, width of fiber bundle: 200 μm, pitch of fiber bundles:500 μm) impregnated with an epoxy resin described above and theplanarizing resin layer 42 formed on the resin-impregnated fiber fabric.

SiO₂ was deposited by evaporation at about 200° C. to a thickness ofabout 100 nm on the surface of the composite layer 43 of which theunevenness was about 120 nm before the formation of the inorganicbarrier layer 45. The unevenness of the resultant surface increased toabout 190 nm. The planarizing resin layer 44 was then formed on theabove surface to a thickness of about 10 μm using the same epoxy resinas that used for the resin with which the glass fiber fabric 3 wasimpregnated and the planarizing resin layer 42. As a result, theunevenness of the surface was reduced to about 90 nm. Note that theplastic substrate had a size of 127 mm×127 nm and roughly the entiresurface was examined for the unevenness. The inorganic barrier layermust be formed at least over the entire display regions, and thus shouldpreferably be formed over the entire surface of the substrate. Theinorganic barrier layer may not be formed on the surface opposite to thesurface facing a liquid crystal layer or an organic EL layer.

As described above, the plastic substrates 20 and 40, of which thesurface unevenness is as small as 100 nm or less, can ensure gooddisplay when they are applied to display devices.

The materials for the plastic substrate suitably used for the displaydevice of the present invention are not limited to those describedabove.

As the transparent resin used for the transparent plastic substrates 20and 40, usable are general transparent resins, thermosetting resins suchas epoxy resins, phenol resins, phenol-epoxy blend resins andbismaleimide-triazine blend resins, and thermoplastic resins such aspolycarbonate, polyethersulfone and polyetherimide.

As the transparent fibers, usable are inorganic fibers such as E glass,D glass and S glass and organic fibers made of a resin such as aromaticpolyimide. The transparent fibers are preferably used in the form offiber bundles, further preferably used in the form of a fabric, asdescribed above.

To improve the mechanical strength of the composite substrate and alsoenhance the uniformity of the mechanical properties and opticalproperties of the composite substrate, the fibers are preferablyarranged uniformly in the plane, the diameter of the fibers and thewidth of the fiber bundles are preferably smaller, and the pitch of thefiber bundles is also preferably smaller. Specifically, the diameter ofeach fiber is preferably about 20 μm or less, more preferably about 10μm. The width of each fiber bundle is preferably 200 μm or less, and thepitch of the fiber bundles is preferably 500 μm or less.

Although a plain-woven fabric is most preferred as the fiber fabric,other general weaves such as satin weave and diagonal weave may beadopted, and even a nonwoven fabric may be used.

The transparency of the plastic substrate is preferably higher.Therefore, to suppress diffuse reflection at the interfaces between thefibers and the resin matrix and scattering at fibers, the fibers and theresin should preferably be selected to have the same refractive index ifpossible. In general, the material of the resin matrix has a wider rangeof selection than the material of the fibers. Also, the resin can bemodified using a substituent in the resin skeleton (for example, therefractive index can be made low with introduction of a fluorine atom,or high with introduction of a bromine atom). Thus, the refractive indexis preferably adjusted by modifying the resin.

The plastic substrate can be fabricated by various known methods usingthe materials for the fibers (fiber bundles and woven fabric) and theresin matrix described above. In the case of using a thermosettingresin, methods such as compression molding, rolling molding, casting andtransfer molding may be employed. In the case of using a thermoplasticresin, methods such as compression, injection molding and extrusion maybe employed.

FIG. 6 diagrammatically shows a liquid crystal display device 200 of anembodiment of the present invention.

The liquid crystal display device 200 includes: a fiber-filledtransparent plastic substrate 20 essentially composed of a compositelayer 23 having the resin-impregnated fiber fabric 3, inorganic barrierlayers 25 for prevention of water vapor transmission formed on thecomposite layer 23, and planarizing resin layers 24 formed on theinorganic barrier layers 25; a plastic substrate 27; and a liquidcrystal layer 26 provided between the plastic substrates 20 and 27. Asthe plastic substrate 27, the same substrate as the plastic substrate 20is preferably used when a transmission or a transmission/reflection(transflective) display device is fabricated. When a reflection liquidcrystal display device is fabricated, in which no transparency isrequired for the plastic substrate 27, another construction may beadopted.

The liquid crystal display device 200 can be fabricated in a generalprocess using the plastic substrate 20 and the plastic substrate 27having the same construction as the plastic substrate 20. For example,TFT elements, a transparent conductive film (ITO) and an alignment filmare formed on one substrate, while color filters, a transparentconductive film (ITO) and an alignment film are formed on the othersubstrate. The transparent conductive film is formed by subjecting ITOto vacuum evaporation at a temperature less than 200° C. (roomtemperature, for example), for example. The alignment film is applied atroom temperature and baked at a temperature less than 200° C. (150° C.to 170° C., for example). A sealant is provided on the substrate havingthe color filters, and spacers are scattered on the substrate having theTFTs. These substrates are then bonded together. A liquid crystalmaterial is then injected into the gap between these substrates byvacuum injection. The surfaces of the substrates of the resultant liquidcrystal display device have unevenness reduced to 100 nm or less, andthus the display quality is prevented from degrading due to variationsin cell thickness.

FIG. 7 diagrammatically shows an organic EL display device 300 of anembodiment of the present invention.

The organic EL display device 300 includes: a fiber-filled transparentplastic substrate 30 essentially composed of a composite layer 33 havingthe resin-impregnated fiber fabric 3, inorganic barrier layers 35 formedon the composite layer 33, and planarizing resin layers 34 formed on theinorganic barrier layers 35; an anode 36 formed on the plastic substrate30; an organic luminescence layer 37 formed on the anode 36; and acathode 38 formed on the organic luminescence layer 37. The plasticsubstrate 30 is substantially the same in construction as the plasticsubstrate 20 described above, and thus the surface thereof hasunevenness reduced to 100 nm or less. The organic EL display device canbe fabricated in a general process. For example, a transparent electrode(ITO) as the anode is formed on the flat surface of the substrate 30.The organic luminescence layer is formed by vacuum evaporation, and atransparent electrode (ITO) as the cathode is formed on the organicluminescence layer by vacuum evaporation. The vacuum evaporation forformation of the anode (hole injection/transport layer), the organicluminescence layer and the cathode (electron injection/transport layer)is performed at a temperature less than 200° C., for example, at roomtemperature. The resultant organic EL display device 300, which has thefiber-filled transparent plastic substrate excellent in surfacesmoothness, can suppress variations in display quality due to unevennessof the substrate surface, and thus can provide high display quality.

The display device of the present invention, provided with the plasticsubstrate, is suitably used for applications that must be light inweight, thin and good in shock resistance, and can providehigher-quality display than conventionally attained. The presentinvention is also applicable to display devices other than the liquidcrystal display devices and the organic EL display devices, such aselectrophoresis display devices.

While the present invention has been described in preferred embodiments,it will be apparent to those skilled in the art that the disclosedinvention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

This non-provisional application claims priority under 35 USC §119(a) onPatent Application No. 2003-377069 filed in Japan on Nov. 6, 2003, theentire contents of which are hereby incorporated by reference.

1. A fabrication method for a display device comprising the steps of:providing a first plastic substrate including a composite layer having aresin-impregnated fiber fabric, an inorganic barrier layer formeddirectly on the resin-impregnated fiber fabric, and a planarizing resinlayer formed on the inorganic barrier layer; and providing a displaymedium layer formed on the planarizing resin layer-side of the firstplastic substrate; wherein the step of providing the first plasticsubstrate includes: preparing the composite layer having theresin-impregnated fiber fabric; forming the inorganic barrier layer on asurface of the resin-impregnated fiber fabric by thin film deposition ata first temperature; forming the planarizing resin layer on theinorganic barrier layer at a second temperature lower than the firsttemperature; and forming a transparent conductive film on theplanarizing resin layer at a third temperature lower than the firsttemperature.
 2. The fabrication method of claim 1, wherein the firsttemperature is 200° C. or more.
 3. The fabrication method of claim 1,wherein the inorganic barrier layer includes silicon dioxide.
 4. Thefabrication method of claim 1, wherein the planarizing resin layer has asurface having an unevenness of about 100 nm or less.
 5. The fabricationmethod of claim 1, wherein the resin-impregnated fiber fabric has anunevenness of about 500 nm to about 600 nm.
 6. The fabrication method ofclaim 1, wherein the inorganic barrier layer is formed on both sides ofthe composite layer.
 7. The fabrication method of claim 1, wherein boththe composite layer and the inorganic barrier layer include unevensurfaces.
 8. The fabrication method of claim 7, wherein the inorganicbarrier layer is formed on both sides of the composite layer.
 9. Thefabrication method of claim 1, wherein both the second temperature andthe third temperature are less than about 200° C.